1. Field of the Invention
This invention relates in general to antenna systems, and in particular to a multi-beam reflector antenna system with a simple beamforming network.
2. Description of Related Art
Communications satellites have become commonplace for use in many types of communications services, e.g., data transfer, voice communications, television spot beam coverage, and other data transfer applications. As such, satellites must provide signals to various geographic locations on the Earth""s surface. As such, typical satellites use customized antenna designs to provide signal coverage for a particular country or geographic area.
The primary design constraints for communications satellites are antenna beam coverage and radiated Radio Frequency (RF) power. These two design constraints are typically thought of to be paramount in the satellite design because they determine which customers on the earth will be able to receive satellite communications service. Further, the satellite weight becomes a factor, because launch vehicles are limited as to how much weight can be placed into orbit.
Many satellites operate over fixed coverage regions and employ polarization techniques, e.g., horizontal and vertical polarized signals, to increase the number of signals that the satellite can transmit and receive. These polarization techniques use overlapping reflectors where the reflector surfaces are independently shaped to produce substantially congruent coverage regions for the polarized signals. This approach is limited because the coverage regions are fixed and cannot be changed on-orbit, and the cross-polarization isolation for wider coverage regions is limited to the point that many satellite signal transmission requirements cannot increase their coverage regions.
Many satellite systems would be more efficient if they contained antennas with high directivity of the antenna beam and had the ability to have the coverage region be electronically configured on-orbit to different desired beam patterns. These objectives are typically met using a phased array antenna system. However, phased array antennas carry with them the problems of large signal losses between the power amplifiers and the beam ports, because of the beamforming network interconnections and long transmission lines. Further, the beamforming network is heavy, difficult to integrate and test, and is difficult to repair or replace without large time and labor costs.
The need to change the beam pattern provided by the satellite has become more desirable with the advent of direct broadcast satellites that provide communications services to specific areas. As areas increase in population, or additional subscribers in a given area subscribe to the satellite communications services, e.g., DirecTV, satellite television stations, local channel programming, etc., the satellite must divert resources to deliver the services to the new subscribers. Without the ability to change beam patterns and coverage areas, additional satellites must be launched to provide the services to possible future subscribers, which increases the cost of delivering the services to existing customers. Further, such systems typically have beamforming networks that are heavy, complex, and difficult to design, test, and integrate onto a spacecraft, and can be difficult to design to produce a uniform performance over a wide scan angle for the antenna.
There is therefore a need in the art for a beamformer that can produce uniform performance over wide scan angles. There is also a need in the art for a beamformer that is easier to integrate and test. There is also a need in the art for a beamforming network that can change the beam pattern on orbit. There is also a need in the art for a beamformer that to provide more complete utilization of space assets without dramatically increasing the cost of manufacturing and operating a satellite.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses an antenna system and method for generating a desired contiguous spot beam pattern, and a signal. The contiguous spot beam generating antenna system comprises a reflector system configured in a side-fed Cassegrain (SFOC) configuration, an array of feed horns comprising at least a subset of feed horns for illuminating the reflector system, and a beamforming network, communicatively coupled to the array of feed horns, for controlling an excitation of the subset of the feed horns in the array of feed horns.
The method comprises illuminating a side fed offset Cassegrain reflector system with an RF signal emanating from an array of feed horns, and controlling an excitation of the subset of the feed horns. The present invention provides a beam former that can produce uniform performance over wide scan angles. The present invention also provides a beamformer that is easier to integrate and test. The present invention also provides a beamforming network that can change the beam pattern on orbit. The present invention also provides a beamformer that to provide more complete utilization of space assets without dramatically increasing the cost of manufacturing and operating a satellite.